1. Field of the Invention
Aspects of the present invention relate to a direct liquid feed fuel cell stack, and more particularly, to a direct liquid feed fuel cell stack having a detachable unit fuel cell module.
2. Description of the Related Art
A direct liquid feed fuel cell generates electricity by electrochemical reactions between an organic fuel, such as methanol or ethanol, and an oxidant, i.e., oxygen in the air. The electricity generated by the direct liquid feed fuel cell has a high specific energy density and current density. Also, since a liquid fuel such as methanol is fed directly to the cell, the direct feed fuel cell does not require a peripheral device such as a fuel reformer, and storing and supplying the liquid fuel is easy.
As depicted in FIG. 1, a unit cell of the direct feed fuel cell has a membrane electrode assembly (MEA) structure having an electrolyte membrane 1 interposed between an anode electrode 2 and a cathode electrode 3. The anode electrode 2 includes a diffusion layer 22 for supplying and diffusing fuel, a catalyst layer 21 at which oxidation reaction of the fuel occurs, and an electrode supporting layer 23. The cathode electrode 3 also includes a diffusion layer 32 for supplying and diffusing the fuel, a catalyst layer 31 at which reduction reaction occurs, and an electrode supporting layer 33.
An electrode reaction of a direct methanol fuel cell (DMFC), which is a type of direct liquid feed fuel cell, includes an anode reaction where fuel is oxidized and a cathode reaction where hydrogen and oxygen are reduced as described below.[Reaction 1]CH3OH+H2O→CO2+6H++6e−  (Anode reaction)[Reaction 2] 3/2O2+6H++6e−→3H2O  (Cathode reaction)[Reaction 3]CH3OH+ 3/2O2→2H2O+CO2  (Overall reaction)
Carbon dioxide, hydrogen ions, and electrons are produced at the anode electrode 2 where the fuel is oxidized (reaction 1). The hydrogen ions migrate to the cathode electrode 3 through a hydrogen ion exchange membrane 1. At the cathode electrode 3, water is produced by the reduction reaction (reaction 2) between hydrogen ions, electrons transferred through an external circuit, and oxygen. Accordingly, water and carbon dioxide are produced as the result of the overall electrochemical reaction (reaction 3) between methanol and oxygen.
The theoretical voltage that can be generated by a unit cell of a DMFC is approximately 1.2V. However, the open circuit voltage at ambient temperature and atmospheric pressure falls below 1 V due to a voltage drop caused by activation overvoltage and ohmic overvoltage. In reality, the actual operating voltage lies in the range of 0.4˜0.6V. Therefore, to obtain higher voltages, a plurality of unit cells are connected in series.
A direct liquid feed fuel cell stack is formed by electrically connecting several unit cells in series and assembling them into a stack. Adjacent unit cells are connected by a conductive bipolar plate 4 between the unit fuel cells. Flow channels 41 and 42 are formed on both sides of the bipolar plate 4, to supply liquid fuel and air to the contacting electrodes.
U.S. Patent Publication No. 2002/0142205 has disclosed a fuel cell stack structure in which a plurality of bipolar plates and MEAs are stacked in series. Unit cells of the fuel cell stack structure are coupled by screws.
However, the fuel cell stack coupled by screws must be unscrewed to replace any unit cell which malfunctions due to manufacturing failure or performance degradation. This activity is time consuming work.